Magnetic alloys



Jan. 19, 1965 CHARLES w. CHEN 3,166,403

MAGNETIC ALLOYS Filed Nov. 16, 1961 2 Sheets-Sheet l AVERAGE MAGNETIC MOMENT PER ATOM BOHR MAGNETONS N 2 3 ATOMIC IN FeCo ALLOY Mn-Fe Co 2.40- BINARY IRON-COBALT AVERAGE MAGNETIC MOMENT PER ATOM, BOHR MAGNETONS N O 9 9 ?;6 26 2 |2s'.4 I zs 26.8 27

Fe Fe co Fe Co ATOMIC N MB R WITNESSES: u E INVENTOR QsmmgQ sh y Charles W. Chen ATTORN Y Jan. 19, 1965 CHARLES w. CHEN 3,166,403

MAGNETIC ALLOYS Filed Nov. 16, 1961 2 Sheets-Sheet 2 Fig.3.

ELECTRICAL RESISTIVITY AT 4.6K, MlCROHM-CM l 2 3 4 5 ATOMIC IN FeCo ALLOY United States Patent Ofifice 3,166,408 Patented Jan. 19, 1965 3,166,408 MAGNETIC ALLOYS Charles W. Chen, Monroeville, Pa., assignor. to Westinghouse Electric Corporation, East Pittsburgh, Pa, a corporation of Pennsylvania Filed Nov. 16, 1961, Ser. No. 153,263 9 Claims. (Cl. 75134) This invention relates to cobalt-iron-manganese alloys having high saturation magnetization which are-capable of being worked into sheet form.

Cobalt-iron alloy magnetic material wherein cobalt and iron are present in substantially equal amounts, with or without small'amounts of additives such as vanadium, has outstanding properties which make the alloys attractive for many applications and particularly for aircraft and military uses. These desirable properties are: (1) a high saturation induction, which makes possible drastic reductions in the size and weight of magnetic cores, (2) rectangular hysteresis loops when the alloy is suitably processed and annealed, which make the alloys applicable in saturable core devices, and (3) high Curie temperatures, which make them useful in elevated temperature environments. p

The superior magnetic properties of cobalt-iron-vanadium alloys, particularly after the magnetic annealing treatment, has been known for some time. However, the saturation magnetization of the cobalt-iron-vanadium alloys are substantially degraded from the like property of the binary cobalt-iron. The binary cobalt-iron alloy has beenrecognized as having the highest knownsaturation magnetization.

g Cobalt-iron-vanadium alloys of thetype discussed above are disclosed in United-States Patent No. 1,862,559, which issued June 14, 1932, to John H. White et al. This patent also discloses small permissive additions of manganese to the ternary alloys without detrimental effect. White et al. does not teach that any significant advantage attaches to manganese additions and his specific alloy examples are confined to manganese in amounts below 0.75% in combination with much larger amounts of vanadium.

Accordingly, it is a primary object of this invention to provide a cobalt-iron-manganese alloy of high purity having a saturation magnetization as high or higher than the binary cobalt-iron alloys and being workable to sheet form.

It is a further object of this invention to provide relatively thin sheets of cobalt-iron-manganese alloys of high purity having a saturation magnetization closely approaching or exceeding that of the binary cobalt-iron alloys.

Another object of this invention is to provide high purity alloys based on the ternary cobalt-iron-manganese alloy system and containing small amounts of at least one element from the group consisting of vanadium, chromium and titanium whereby a material having a high saturation magnetization and good electrical resistivity is provided.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

For a better understanding of the nature and objects of the invention, reference may be had to the following detailed description and to the figures, in which:

FIGURE 1 is a series of curves in which the average magnetic moment per atom of cobalt-iron base alloys is plotted against atomic percent of addition elements for a number of addition elements, and

- FIG. 2 is a graph in which the average magnetic mo-' ment per atom is plotted against atomic number for ironcobalt-manganese alloys with different iron-cobalt ratios.

FIG. 3 is a graph in which the electrical resistivity of a plurality of cobalt-iron base alloys is plotted against the weight percent of certain alloying additions.

This invention is directed to cobalt-iron-manganese magnetic alloys of high purity containing, by weight, from 3.2% to 10% manganese, from 35% to cobalt, and the balance at least 35% iron and small amounts of incidental impurities including detrimental impurities not exceeding 200 parts per million. It has further been discovered that while the whole range of alloys has a relatively high saturation magnetization when subjected to a final heat treatment for producing optimum magnetic properties, the range includes alloy compositions yielding a higher saturation induction than any known hitherto including the binary cobalt-iron alloys which have been considered to possess the highest saturation magnetization when properly treated.

A preferred range of magnetic alloys of high purity consists of, by weight, from 3.2% to 10% manganese, and the balance substantially equal amounts of iron and cobalt with small amounts of incidental impurities.

A still more specific range of preferred alloys of high purity displaying exceptionally high saturation induction consists of, by weight, from 3.2% to 6% manganese, and the balance substantially equal amounts of iron and cobalt with small amounts of incidental impurities.

A high purity magnetic alloy which has displayed a higher saturation induction than any previously known magnetic alloy consistsof, by weight, about 4.5% manganese, and the balance substantially equal amounts of iron and cobalt with small amounts of incidental impurities.

Alloys based on the ternary alloys but containing at least one additional element, which display high electrical resistivity in addition to high saturation magnetization, also form a part of this invention. Such high purity alloys include from about 3% to 10% by weight of manganese, at least one element selected from the group consisting of vanadium, titanium and chromium, vanadium when present not exceeding 4% by weight, titanium when present not exceeding 4% by weight, chromium when present not exceeding 2% by weight, at least 35% by weight cobalt and the balance at least 35% by weight iron and small amounts of incidental impurities. Preferred alloys of this type contain substantially equal amounts of iron and cobalt.

By substantially equal amounts of iron and cobalt, it is meant that the quantities of iron and cobalt in these alloys are equal within a tolerance of :3

In alloys of the type described herein it is generally desired to achieve a high degree of purity. Therefore, raw materials of high purity are employed and vacuum melting or melting under a protective atmosphere is used to prevent contamination and to further reduce impurities in the alloys. Nevertheless, small amounts of elements such as nickel (which may be present in amounts of the order of 0.1%), oxygen, nitrogen, sulfur and phosphorus may be present.

Based on certain original theoretical work, it was predicted that manganese alone among the group of metals comprising titanium, vanadium, chromium, manganese, nickel, molybdenum, tungsten, niobium, rhenium and copper would be capable of increasing the average magnetic moment per atom of the ternary alloys which consist primarily of equal amounts of cobalt and iron.

The following table is based on theoretical considerations and indicates the effect of increasing manganese concentration upon the average magnetic moment per atom of the alloys as compared with the basic cobalt-iron binary alloy.

35 4 TABLE I Atomic Wt. Atomic Wt. Atomic Wt. Atomic Wt. Atomic Wt. Atomic Wt. Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent M 0 2 1.9 4 3.8 8 t 7.7 12 11.6 16 15.2 Fe 50 48. 7 49 47. 7 48 46. 8 46 44.9 44 43. 0 42 41. 1

Aver. Magnetic Moment per Atom, Bohr Magneton Following the lead indicated by the theoretical study a number of samples of alloys of varying compositions were prepared. The composition of these alloys and related data are given in the following Table II.

TABLE II Saturation magnetization of Fe-Co and Fe-Co-Mn alloys Weight Percent Satura- Type of Bohr tion 1 Spec. No. Alloy Fe/Co Mag- Magneti- Fe Co Mn, neton zation. Gauss 41 .4/1 79.1 20. 9 0 2. 39 24,000 45.. 4/1 75.3 19. 9 4. 8 2. 25 22, 600 46.. 4/1 71. 4 l8. 8 9. 8 2. 09 20, 900 57.. 2/1 65. 34. 5 0 2. 44 24, 800 48.. Ternary--. 2/1 62.3 32. 9 4. 8 2. 36 23, 900 50 o 2/1 59.1 31. 2 9. 7 2. 28 22, 900 43 Binary.... 1/1 48.7 51. 3 0 2. 39 24, 600 51. Ternary..- 1/1 46. 8 49. 4 3. 8 2. 45 25,100 52- 1/1 45. 8 48. 4 5.8 2. 36 24, 200 53. 1/1 44. 9 47. 4 7. 7 2. 42 24, 700 54. ill 44. O 46. 4 9. 6 2. 39 24, 300 44. 2/3 38. 7 61. 3 0 2. 30 23, 800 55 Ternary..- 2/3 36. 8 58. 4 4. 8 2. 33 24-, 000 56 o 2/3 35.0 55.4 9.6 2.37 24, 200

1 Based on estimated densities.

As will be seen from the above table and from FIG. 2, manganese increases the average magnetic moment of those ternary alloys which contain iron and cobalt atoms in the ratio of 1:1 or 2:3 but not at 2:1 or 4:1. Thus, manganese exerts its beneficial effect on the saturation of iron-cobalt alloys in the range just beyond the composition (35 atomic percent cobalt) at which the peak value of saturation is attained in the binary alloys. The curves of FIG. 2 were obtained with material in the partially ordered condition which yields relatively high values of average magnetic moment per atom. The material in the disordered condition would yield curves at somewhat lower values, but during working of the alloys it is preferable to have the material in the disordered state. After working, an ordered state can be produced in the alloy to maximize the saturation magnetization obtainable.

Considering the iron-cobalt-manganese alloy in which the iron to cobalt ratio is 1:1, up to about 4.5% by weight of manganese may be included to obtain higher saturation magnetization. At higher manganese concentrations, precipitation of the high temperature phase, which is non-magnetic or only weakly magnetic, significantly affects the alloys and brings about a decrease ofsaturation magnetization. For alloys in which the iron to cobalt ratio is 2:3, however, as much as 10% by weight manganese may be included in the alloy to effect an increase in saturation magnetization. A 10% manganese-45% iron45% cobalt alloy (9.6% manganese44% iron-46.4% cobalt in weight percent) retains the same saturation magnetization as the binary alloy iron-cobalt (see sample 54 of Table II, and FIGURE 2). Although such an alloy does not have the maximum saturation magnetization which is displayed by the 4.5% manganese alloy it does have certain other compensating advantages:

(:1) Higher electrical resistance.

(b) Lower cost due to the lower concentration of cobalt.

Thus, the iron-cobalt-ma'nganese system provides a wide spectrum of magnetic alloys offering saturation magnetization as high or higher than that of the binary iron-cobalt alloys and other advantages, particularly in-' creased electrical resistivity and lower cost. In working the alloys of this invention to sheet form precautions similar to those observed in working iron-cobalt-vanadium alloys are necessary. For example, a rapid quench in brine and ice from a temperature of about 1000 C. will produce the martensitic structure which confers sufficient ductility on the alloy so that cold rolling to final thickness is possible. Thereafter, maximum magnetic H properties may be developed by appropriate heat treatment.

An important consideration in many applications for the cobalt-iron base alloys, is the electrical resistivity of such alloys, since the higher the resistivity, the lower are the electrical losses sustained in operation of the devices incorporating the alloys. At this point, it is Well to examine FIG. 3, in which the effect of various alloying ingredients on the electrical resistivity of the basic cobalt-iron base alloys can readily be perceived. It will be noted that manganese increases the electrical resistivity of the coba1=t-iron alloy only slightly, While vanadium, chromium and titanium are many times as effective in increasing electrical resistivity. Accordingly, it is within the scope of this invention to add one or more of these latter elements to cobalt-iron-manganese alloys to increase electrical resistivity. Vanadium may be present in these alloys in amounts of up to 4%, by weight; chromium may be present in amounts of up to 2%, by weight; titanium may be present in amounts of up to 4%, by weight. Combination of two or more of these elements may be employed to increase electrical resistivity, but it is preferred to limit the total additions of these elements to 2%, by weight.

Certain preferred alloys of this type have been prepared. One such alloy (Alloy II) is composed of, by weight, about 3%, manganese, about 1% vanadium and the balance essentially equal amounts of cobalt and iron.

Another such alloy (Alloy III) is composed of, by

weight, about 3.5% manganese, about 0.75% chromium, and the balance essentially equal amounts of cobalt and 11'011.

' In the following table the electrical resistivities at room temperature and at the temperature of liquid helium (absolute electrical resistivity) of Alloy II and All-0y III are compared with the same characteristics of a binary cobalt-iron alloy (Alloy I) in which cobalt and iron are present in substantially equal amounts:

From the above table it will be observed that moderately small additions of vanadium and chromium produce large increases in the electricial resistivity of cobaltiron-manganese alloys. Similar results. are produced by additions of titanium. Thus, by judicious selection of additions to the cobalt-iron-manganese alloys, materials having both high saturation magnetization and good electrical resistivity can be made.

Because of the importance of, high purityin obtaining the desired properties in the magnetic'alloys of this invention, the iron, cobalt, manganese and other alloying constituents used were all of the highest purity electrolytic grades commercially available. However, even in materials of such high purity there invariably exist small amounts of impurities. Thus, nickel may be present in amounts of up to as much as 0.1% because it is most difficult to separate it from cobalt with which it is always associated in the ores thereof. In any case, nickel is not seriously objectionable as an impurity at this level. Similarly, small or trace amounts of the detrimental elements carbon, sulfur, and phosphorus will be present in the alloys, but the total amount of these latter three elements ordinarily will not exceed 200 parts per million.

In making the alloys of this invention any convenient heating means such as induction or are melting may be employed, so long as a protective atmosphere; i.e., vacuum, argon, helium or nitrogen, is provided. Hydrogen should be avoided since it fosters embrittlement. The level of purity of the alloys should be maintained at as high a level as practical, for oxygen, hydrogen, sulfur and phosphorus tend to embrittle the alloy and may degrade the magnetic properties. Forging or hot rolling may be used to reduce the ingot to slab or billet form of a thickness suitable for cold rolling. Prior to forging or hot rolling the ingot is preheated in a protective atmosphere at a temperature above 1000 C. Prolonged heating or excessively high temperatures should be avoided, since thorough heating is all that is required, and thu thickness of the ingot essentially determines the length of time By controlled heating, grain coarsening is avoided with its deleterious effect on the physical properties of the alloy.

One satisfactory melting and processing treatment is described hereafter. Ingots of the manganese-iron-cobalt alloys are vacuum induction melted in a magnesium oxide crucible, with a low partial pressure helium atmosphere providing a subatmospheric pressure environment. The charge is placed in the crucible prior to heating, and the vacuum is maintained until just before the charge melts, at which time a small pressure of helium is introduced into the furnace space. The ingots are cast in slab mold to eliminate the need for any forging operation prior to hot rolling. The hot top of each ingot is removed, leaving a slab suitable for further processing. A portion of each slab is then hot rolled to a strip of desired thickness, for example, approximately 90 mils. It is preferable to carry out the hot rolling without reheating intermediate the rolling step in order to minimize grain growth. However, in some cases the ingots may be heated to a temperature of about 1000 C. prior to hot rolling in either a protective argon or helium atmosphere.

After hot rolling, the strip is quenched in a medium below room temperature, such as brine and ice. After quenching, the strips are prepared for cold rolling by pickling in a solution of hydrochloric and nitric acids until clean. Finally, the strips are cold rolled to sheets of the desired thickness, for example, from 1 to mils.

The thin sheet is deburred, degreased, and insulated with magnesium oxide and then wound into toroidal cores. The cores may then be annealed in a dry hydrogen atmosphere having a dew point of 45 C. or lower. The anneal may be carried out at temperatures above 1000 C. for a sufficient length of time to eliminate nonequilibrium structures. After annealing, the cores are cooled at a controlled rate to room temperature. The cooling may be done either with or without a magnetic field.

The amount of reduction in the hot and cold working stages is not critical, the most important consideration during rolling being convenience in handling the material and arriving at the desired final thickness.

The alloy sheets of this invention can be readily duplicated. The alloys described 'may be cold worked to sheets and retain sufiicient ductilityafter cold working to enable fabrication ofthe sheets into the form necessary for utilizing the magnetic alloy as a component of electrical apparatus. The alloy sheets of this invention can be applied to the magnetic core structures of transformers and rotating equipment and are capable of significantly improving performance of such structures. The high operating inductions possible with the use of these alloy sheets permit the reduction of the size and weight of inductive components. Reductions in size of as much as 25% can be achieved, a feature which is of particular importance in the design of aircraft components. Alloy sheets made of the alloy of this invention will have good, high, temperature properties and will be capable of functioning in environments Where the performance of most magnetic alloys will be seriously degraded.

Since obviou modifications may be made in the invention without departing from the scope thereof, it is intended that all matter contained in the above description or taken in connection with the accompanying figures shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. A magnetically so-ft alloy of high purity consisting of, by weight, from 3.2% to 10% manganese, from 35% to 60% cobalt, and the balance at least 35% iron and small amounts of incidental impurities, including detrimental impuritie not exceeding 200 parts per million.

2. A magnetically soft alloy of high purity consisting of, by weight, from 3.2% to 10% manganese, and the balance substantially equal amounts of iron and cobalt with small amounts of incidental impurities.

3. A magnetically soft alloy of high purity consisting essentially of, by weight, from 3.2% to 6% manganese, and the balance substantially equal amount of iron and cobalt with small amounts of incidental impurities, the alloy having a high saturation magnetization.

4. A magnetically soft alloy of high purity consisting of, by weight, about 4.5%. manganese, and the balance substantially equal amounts of iron and cobalt with small amounts of incidental impurities, the alloy having a high saturation magnetization.

5. A magnetically soft alloy of high purity consisting essentially of, by weight, from about 3% to 10% manganese, at least one element selected from the group consisting of vanadium, titanium and chromium, vanadium when present not exceeding 4%, titanium when present not exceeding 4%, chromium when present not exceeding 2%, at least 35% cobalt, and the balance iron and small amounts of incidental impurities, including detrimental impurities not exceeding 200 parts per million.

6. The alloy of claim 5 wherein the total amount of vanadium, titanium and chromium does not exceed 2% by weight.

7. The alloy of claim 6 wherein the cobalt and iron are present in substantially equal amounts.

8. A high purity magnetically soft alloy composed of, by weight, about 3% manganese, about 1% vanadium and the balance substantially equal amounts of cobalt and iron, and small amounts of incidental impurities, the alloy exhibiting relatively high saturation magnetization and high electrical resistivity.

9. A high purity magnetically soft alloy composed of, by weight, about 3.5% manganese, about 0.75% chromium, and the balance essentially equal amounts of cobalt and iron, and small amounts of incidental impurities, the alloy exhibiting high saturation magnetiza- FOREIGN PATENTS tion and high electrlcal resistivity. 571,676 G Britain Sept 4 1945 References Cited in the file of this patent OTHER REFERENCES v UNITED STATESPATENTS 5 v Bozorth: Ferromagnetism, D. Van Nostrand'COm- 1,715,647 Elmen June 4 1.929 pany, Inc; New York, 1951, pages 20 0 and 432 to 434. 1,715,648 Elmen June 4,1929 References Cited by the Applicant 1,739,752 Elmen Dec. 17, 192 9 1 1,862,559 White et a1. June 14, 1932 FOREIGN 1?A' TENTS 1,927,940 Koster Sept. 26, 1933 10 558,738 1/ 44 'GTeatBmamv 3,024,141 Burket Mar. 6 1962 

1. A MAGNETICALLY SOFT ALLOY OF HIGH PURITY CONSISTING OF, BY WEIGHT, FROM 3.2% TO 10% MANGANESE, FROM 35% TO 60% COBALT, AND THE BALANCE AT LEAST 35% IRON AND SMALL AMOUNTS OF INCIDENTAL IMPURITIES, INCLUDING DETRIMENTAL IMPURITIES NOT EXCEEDING 200 PARTS PER MILLION. 